Power consumption in motion-capture systems with audio and optical signals

ABSTRACT

The technology disclosed provides systems and methods for reducing the overall power consumption of an optical motion-capture system without compromising the quality of motion capture and tracking. In implementations, this is accomplished by operating the motion-detecting cameras and associated image-processing hardware in a low-power mode (e.g., at a low frame rate or in a standby or sleep mode) unless and until touch gestures of an object such as a tap, sequence of taps, or swiping motions are performed with a surface proximate to the cameras. A contact microphone or other appropriate sensor is used for detecting audio signals or other vibrations generated by contact of the object with the surface.

RELATED APPLICATION

The application claims the benefit of U.S. provisional Patent Application No. 61/750,007, entitled, “OBJECT DETECTION AND TRACKING WITH AUDIO AND OPTICAL SIGNALS,” filed on Jan. 8, 2013. The provisional application is hereby incorporated by reference for all purposes.

FIELD OF THE TECHNOLOGY DISCLOSED

The present disclosure relates generally to imaging systems and in particular to three-dimensional (3D) object detection, tracking and characterization.

BACKGROUND

Motion capture has numerous applications. For example, in filmmaking, digital models generated using motion capture can be used as the basis for the motion of computer-generated characters or objects. In sports, motion capture can be used by coaches to study an athlete's movements and guide the athlete toward improved body mechanics. In video games or virtual reality applications, motion capture facilitates interaction with a virtual environment in a natural way, e.g., by waving to a character, pointing at an object, or performing an action such as swinging a golf club or baseball bat. The term “motion capture” refers generally to processes that capture movement of a subject in three-dimensional (3D) space and translate that movement into, for example, a digital model or other representation.

Motion-capture systems may utilize one or more cameras to capture sequential images of an object in motion, and computers to analyze the images to reconstruct an object's shape, position, and orientation as a function of time. For 3D motion capture, at least two cameras are typically used. In order to accurately track motion in real or near-real time, the camera(s) of motion-capture systems typically operate at a frame rate of at least 15 image frames per second.

Unfortunately, however, image acquisition at such high rates entails significant power requirements, which can be exacerbated by the need for adequate lighting to facilitate image capture. Power requirements can pose a practical limit to the range of applications of motion-capture systems, as excessive power consumption can render their employment impractical or economically infeasible.

What is really needed are techniques that leverage the mechanical impact of a touch to facilitate low-power operational modes unavailable with purely optical systems. For example, a touch may “wake up” a device from a low-power standby mode using an electrical signal from a piezo or electromagnetic actuator.

SUMMARY

The technology disclosed provides systems and methods for reducing the overall power consumption of an optical motion-capture system without compromising the quality of motion capture and tracking In one implementation, this is accomplished by operating the motion-detecting cameras and associated image-processing hardware in a low-power mode (e.g., at a low frame rate or in a standby or sleep mode) unless and until touch gestures of an object such as a tap, sequence of taps, or swiping motions are performed with a surface proximate to the cameras. A contact microphone, or other vibratory or acoustical sensor, is used for detecting audio signals (or other vibrational phenomena) generated by contact of the object with the surface. In one implementation, the contact microphone is a surface microphone such as a piezo or electromagnetic actuator. In another implementation, the audio signals are filtered to exclude frequencies outside a defined range. In yet another implementation, the motion-capture system includes an ultra-low-power circuit responsive to low-voltage audio signals.

Once the object's contact with the surface is detected, the motion-capture system is “woken up,” i.e., switched into a high-power mode, in which it acquires and processes images at a frame rate sufficient for accurate motion tracking. For example, in a computer system that interprets a user's hand gestures as input, the requisite motion-capture system associated with the computer terminal may be idle, i.e., run in a low-power mode, as long as everything in the camera's field of view is static, and start capturing images at a high rate only when an electrical signal generated, by a piezo or electromagnetic element, directly from a touch or tap on the surface. Because the gesture directly causes generation of the electrical signal, active, power-consuming monitoring circuitry is unnecessary. Accordingly, in some implementations high power consumption is limited to time periods when it results in a corresponding benefit, i.e., when the system is actively used for motion capture.

Advantageously, some embodiments can obviate the need for active monitoring circuitry because mechanical action directly produces the signal, active monitoring circuitry is unnecessary, and power requirements are therefore minimized. By contrast, in conventional approaches, some form of active monitoring is typically necessary to produce recognize optical gestures, meaning that, for the system to be able to recognize “wake up” gestures, at least some amount of power is constantly consumed.

Other aspects and advantages of the present technology can be seen on review of the drawings, the detailed description and the claims, which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, with an emphasis instead generally being placed upon illustrating the principles of the technology disclosed. In the following description, various implementations of the technology disclosed are described with reference to the following drawings, in which:

FIG. 1 illustrates a system for capturing image and audio data according to an implementation of the technology disclosed.

FIG. 2 is a simplified block diagram of a computer system implementing an image analysis apparatus according to an implementation of the technology disclosed.

FIG. 3A is a perspective view from the top of a sensor in accordance with the technology disclosed, with microphones along an edge surface thereof.

FIG. 3B is a perspective view from the bottom of a sensor in accordance with the technology disclosed, with microphones along the bottom surface thereof.

FIG. 3C is a perspective view from the top of a sensor in accordance with the technology disclosed, with detachable microphones configured for placement on a surface.

FIG. 4 shows a flowchart of one implementation of operating a motion-capture system with reduced power consumption using audio and optical signals.

DETAILED DESCRIPTION

Implementations of the technology disclosed relate to methods and systems operating a motion-capture system with reduced power consumption using audio signals. For example, a sequence of images can be correlated to construct a 3-D model of the object, including its position and shape. A succession of images can be analyzed using the same technique to model motion of the object such as free-form gestures. In low-light situations, where free-form gestures cannot be recognized optically with a sufficient degree of reliability, audio signals can supply the direction and location of the object as further described herein.

Refer first to FIG. 1, which illustrates a system 100 for capturing image and audio data according to one implementation of the technology disclosed. System 100 includes a pair of cameras 102, 104 coupled to an image and audio processing system 106. Cameras 102, 104 can be any type of camera, including cameras sensitive across the visible spectrum or with enhanced sensitivity to a confined wavelength band (e.g., the infrared (IR) or ultraviolet bands); more generally, the term “camera” herein refers to any device (or combination of devices) capable of capturing an image of an object and representing that image in the form of digital data. For example, line sensors or line cameras rather than conventional devices that capture a two-dimensional (2D) image can be employed. The term “light” is used generally to connote any electromagnetic radiation, which may or may not be within the visible spectrum, and may be broadband (e.g., white light) or narrowband (e.g., a single wavelength or narrow band of wavelengths).

Cameras 102, 104 are preferably capable of capturing video images (i.e., successive image frames at a constant rate of at least 15 frames per second), although no particular frame rate is required. The capabilities of cameras 102, 104 are not critical to the technology disclosed, and the cameras can vary as to frame rate, image resolution (e.g., pixels per image), color or intensity resolution (e.g., number of bits of intensity data per pixel), focal length of lenses, depth of field, etc. In general, for a particular application, any cameras capable of focusing on objects within a spatial volume of interest can be used. For instance, to capture motion of the hand of an otherwise stationary person, the volume of interest might be defined as a cube approximately one meter on a side.

The illustrated system 100 also includes a pair of microphones 108, 110 coupled to the image and audio processing system 106. Microphones 108, 110 can be any type of microphone useful for obtaining audio signals from a surface, e.g., microphones that use electromagnetic induction (dynamic microphones), capacitance change (condenser microphones), piezoelectric response, or light modulation to produce an electrical signal from mechanical vibration; more generally, the term “microphone” herein refers to any device (or combination of devices) capable of converting mechanical vibration into an electrical signal.

Microphones 108, 110 are preferably capable of capturing audio signals as they propagate through a surface; for example, the microphones 108, 110 may be contact microphones. Such devices, otherwise known as pickups or piezos, are designed to sense audio vibrations through solid objects. Unlike normal air microphones, contact microphones have little sensitivity to air vibrations and transduce only structure-borne sound. Commonly available contact microphone elements include a thin piezoelectric ceramic round glued to a thin brass or alloy metal disc. More recently, flexible PVDF fluoropolymer piezo films have been used as contact microphones.

In operation, cameras 102, 104 are oriented toward a region of interest 112, that includes at least a portion of a surface 116, in which an object of interest 114 (in this example, a hand) moves across and in contact with the surface 116 along the indicated path 118. The microphones 108, 110 are positioned for contact with surface 116 for capturing audio signals propagating there through. In some implementations, one or more light sources 115, 117 are arranged to illuminate the region of interest 112. In some implementations, one or more of the cameras 102, 104 are disposed opposite the motion to be detected, e.g., where the hand 114 is expected to move. This is an optimal location because the amount of information recorded about the hand is proportional to the number of pixels it occupies in the camera images, and the hand will occupy more pixels when the camera's angle with respect to the hand's “pointing direction” is as close to perpendicular as possible. Image and audio processing system 106, which can be, e.g., a computer system, can control the operation of microphones 108, 110 and cameras 102, 104 to capture images of the region of interest 112 and audio signals propagating through surface 116. Based on the captured images and audio signals, image and audio processing system 106 determines the position and/or motion of object 114.

For example, as a step in determining the motion of object 114, image and audio processing system 106 can determine which pixels of various images captured by cameras 102, 104 contain portions of object 114. In some implementations, any pixel in an image can be classified as an “object” pixel or a “background” pixel depending on whether that pixel contains a portion of object 114 or not. Object pixels can thus be readily distinguished from background pixels based on brightness. Further, edges of the object can also be readily detected based on differences in brightness between adjacent pixels, allowing the position of the object within each image to be determined. In some implementations, the silhouettes of an object are extracted from one or more images of the object that reveal information about the object as seen from different vantage points. While silhouettes can be obtained using a number of different techniques, in some implementations, the silhouettes are obtained by using cameras to capture images of the object and analyzing the images to detect object edges. Correlating object positions between images from cameras 102, 104 allows image and audio processing system 106 to determine the location in 3D space of object 114, and analyzing sequences of images allows image and audio processing system 106 to reconstruct 3D motion of object 114 using motion algorithms or other techniques. See, e.g., U.S. patent application Ser. No. 13/414,485 (filed on Mar. 7, 2012) and U.S. Provisional Patent Application Nos. 61/724,091 (filed on Nov. 8, 2012) and 61/587,554 (filed on Jan. 7, 2012), the entire disclosures of which are hereby incorporated by reference.

For example, in low-light situations where free-form gestures cannot be recognized optically with a sufficient degree of reliability, system 106 may switch to a touch mode in which touch gestures are recognized. Alternatively, system 106 may switch to the touch mode, or supplement image capture and processing with touch sensing, when signals from microphones 108, 110 are sensed. In still another operational mode, a tap or touch gesture may act as a “wake up” signal to bring the image and audio analysis system 106 from a standby mode to an operational mode. For example, the system 106 may enter the standby mode if optical signals from the cameras 102, 104 are absent for longer than a threshold interval. Because the microphones 108, 110 can generate detectable electrical signals without external power, the system 106 may contain an ultra-low-power wake-up circuit such as those designed for low-voltage detection (for example, the Microchip AN879 module). This approach provides significant power savings relative to a system requiring an optical wake-up signal (such as a gesture), since optical detectors, such as cameras 102, 104, must remain in an active state if wake-up gestures are to be registered.

In determining the entrance of object 114 in region of interest 112, image and audio processing system 106 can determine its presence by computationally analyzing the audio signals captured by microphones 108, 110. For example, any part of object 114 that comes in contact with surface 116 can be classified as a “source” of the audio signals captured. The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. For example, surface 116 may be a solid material—e.g., one with enough surface texture to generate contact noise when rubbed. In some implementations, audio signals outside a defined range will be excluded from analysis by a filter to remove background noise. Once again, the properties of the medium as identified by signal matching can be used to define the characteristics of a suitable filter, which may be implemented in software or controllable hardware by image and audio processing system 106.

Refer now to FIG. 2, which shows a simplified block diagram of a computer system 200 for implementing image and audio processing system 106. Computer system 200 includes a processor 202, a memory 204, a microphone and camera interface 206, a display 208, speakers 209, a keyboard 210, and a mouse 211. Memory 204 can be used to store instructions to be executed by processor 202 as well as input and/or output data associated with execution of the instructions. In particular, memory 204 contains instructions, conceptually illustrated as a group of modules described in greater detail below, that control the operation of processor 202 and its interaction with the other hardware components. An operating system directs the execution of low-level, basic system functions such as memory allocation, file management and operation of mass storage devices. The operating system may be or include a variety of operating systems such as Microsoft WINDOWS operating system, the Unix operating system, the Linux operating system, the Xenix operating system, the IBM AIX operating system, the Hewlett Packard UX operating system, the Novell NETWARE operating system, the Sun Microsystems SOLARIS operating system, the OS/2 operating system, the BeOS operating system, the MACINTOSH operating system, the APACHE operating system, an OPENSTEP operating system, iOS, Android or other mobile operating systems, or another operating system of platform.

The computing environment may also include other removable/non-removable, volatile/nonvolatile computer storage media. For example, a hard disk drive may read or write to non-removable, nonvolatile magnetic media. A magnetic disk drive may read from or writes to a removable, nonvolatile magnetic disk, and an optical disk drive may read from or write to a removable, nonvolatile optical disk such as a CD-ROM or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The storage media are typically connected to the system bus through a removable or non-removable memory interface.

Processor 202 may be a general-purpose microprocessor, but depending on implementation can alternatively be a microcontroller, peripheral integrated circuit element, a CSIC (customer-specific integrated circuit), an ASIC (application-specific integrated circuit), a logic circuit, a digital signal processor, a programmable logic device such as an FPGA (field-programmable gate array), a PLD (programmable logic device), a PLA (programmable logic array), an RFID processor, smart chip, or any other device or arrangement of devices that is capable of implementing the steps of the processes of the technology disclosed.

Microphone and camera interface 206 can include hardware and/or software that enables communication between computer system 200 and cameras 102, 104, as well as microphones 108, 110 (see FIG. 1). Thus, for example, microphone and camera interface 206 can include one or more camera data ports 216, 218 and microphone ports 217, 219 to which the cameras and microphones can be connected (via conventional plugs and jacks), as well as hardware and/or software signal processors to modify data signals received from the cameras and microphones (e.g., to reduce noise or reformat data) prior to providing the signals as inputs to a motion-capture (“mocap”) program 214 executing on processor 202. In some implementations, microphone and camera interface 206 can also transmit signals to the cameras and microphones, e.g., to activate or deactivate them, to control camera settings (frame rate, image quality, sensitivity, etc.), to control microphone settings (calibration, sensitivity levels, etc.), or the like. Such signals can be transmitted, e.g., in response to control signals from processor 202, which may in turn be generated in response to user input or other detected events.

Instructions defining mocap program 214 are stored in memory 204, and these instructions, when executed, perform motion-capture analysis on images supplied from cameras and audio signals from microphones connected to microphone and camera interface 206. In one implementation, mocap program 214 includes various modules, such as an object analysis module 222 and a path analysis module 224. Object analysis module 222 can analyze images (e.g., images captured via interface 206) to detect edges of an object therein and/or other information about the object's location. Object analysis module 222 can also analyze audio signals (e.g., audio signals captured via interface 206) to localize the object by, for example, triangulation as discussed above. Path analysis module 224 can track and predict object movements in 3D based on information obtained via the cameras. Audio signals place the object on a known surface, and the strength and variation of the signals can be used to detect object's presence. Successive camera images are analyzed at the pixel level to extract object movements and velocities. If both audio and image information is simultaneously available, both types of information can be analyzed and reconciled to produce a more detailed and/or accurate path analysis.

Display 208, speakers 209, keyboard 210, and mouse 211 can be used to facilitate user interaction with computer system 200. These components can be of generally conventional design or modified as desired to provide any type of user interaction. In some implementations, results of motion capture using microphone and camera interface 206 and mocap program 214 can be interpreted as user input. For example, a user can perform hand gestures or motions across a surface that are analyzed using mocap program 214, and the results of this analysis can be interpreted as an instruction to some other program executing on processor 200 (e.g., a web browser, word processor, or other application). Thus, by way of illustration, a user might use upward or downward swiping gestures to “scroll” a webpage currently displayed on display 208, to use rotating gestures to increase or decrease the volume of audio output from speakers 209, and so on. Path analysis module 224 may represent the detected path as a vector and extrapolate to predict the path, e.g., to improve rendering of action on display 208 by anticipating movement.

It will be appreciated that computer system 200 is illustrative and that variations and modifications are possible. Computer systems can be implemented in a variety of form factors, including server systems, desktop systems, laptop systems, tablets, smart phones or personal digital assistants, and so on. A particular implementation may include other functionality not described herein, e.g., wired and/or wireless network interfaces, media playing and/or recording capability, etc. In some implementations, one or more cameras and two or more microphones may be built into the computer rather than being supplied as separate components. Further, an image or audio analyzer can be implemented using only a subset of computer system components (e.g., as a processor executing program code, an ASIC, or a fixed-function digital signal processor, with suitable I/O interfaces to receive image data and output analysis results).

While computer system 200 is described herein with reference to particular blocks, it is to be understood that the blocks are defined for convenience of description and are not intended to imply a particular physical arrangement of component parts. Further, the blocks need not correspond to physically distinct components. To the extent that physically distinct components are used, connections between components (e.g., for data communication) can be wired and/or wireless as desired. Thus, for example, execution of object analysis module 222 by processor 202 can cause processor 202 to operate microphone and camera interface 206 to capture images and/or audio signals of an object traveling across and in contact with a surface to detect its entrance by analyzing the image and/or audio data.

FIGS. 3A-3C illustrate three different configurations of system 100, all packaged within a single housing as an integrated sensor. In all cases, sensor 300A, 300B, 300C includes a top surface 305, a bottom surface 307, and a side wall 310 spanning the top and bottom surfaces 305, 307. With reference also to FIG. 1, the top surface 305 of sensor 300A contains a pair of windows 315 for admitting light to the cameras 102, 104, one of which is optically aligned with each of the windows 315. If the system includes light sources 115, 117, surface 305 may contain additional windows for passing light to the object(s) being tracked.

In sensor 300A, microphones 108, 110 are located on the side wall 310. Desirably, the microphones are flush with the surface of side wall 310 so that, with this surface in contact with a table or other bearing surface, the microphones are in contact with the bearing surface. Of course, the microphones can be recessed slightly from side wall 310 in order to prevent frictional damage so long as acoustic coupling to the bearing surface remains adequate. This configuration allows the sensor 300A to be positioned to face objects in contact with and traveling across the bearing surface, so that tracking can be based simultaneously on optical and audio signals.

In sensor 300B, microphones 108, 110 are located on the bottom surface 307, once again in a flush or slightly recessed configuration. The top surface of the sensor 300B (not shown in the figure) contains camera windows 315 as shown in FIG. 3A. This arrangement is well-suited to applications in which the sensor is primarily used for optical tracking above the bearing surface, and audio tracking based on movements along and in contact with the bearing surface represent an alternative operational mode—e.g., the user may tap the bearing surface in order to “wake up” sensor 300B from a standby mode.

In FIG. 3C, microphones 108, 110 are external contact transducers that connect to sensor 300A via audio jacks 320. This configuration permits the microphones to be located away from the sensor, e.g., if the sensor does not rest on the surface with which tracked objects make contact.

It will be appreciated that the figures shown in FIGS. 3A-3C are illustrative. In some implementations, it may be desirable to house the system 100 in a differently shaped enclosure or integrated within a larger component or assembly. Furthermore, the microphones are depicted as small cylindrical discs, but neither the size nor the shape is critical.

FIG. 4 shows a flowchart 400 of one implementation of operating a motion-capture system with reduced power consumption using audio and optical signals. Initially, the control system 106 operates the cameras in a low-power mode (step 402), such as a standby or sleep mode where motion capture does not take place at all or a slow image-acquisition mode (e.g., with image-acquisition rates of five frames per second or less). This does not only reduce power consumption by the cameras, but typically also decreases the power consumption of the control and image-processing system 106, which is subject to a lower processing burden as a consequence of the decreased (or vanishing) frame rate. While the system is in low-power mode, the control system 106 monitors a contact microphone that is coupled to the motion-capture system and in contact with a surface that a user touches.

As long as no audio signals are detected by the contact microphone, the system continues to operate in low-power mode and the contact microphone continues to be monitored. Once an audio signal is detected (step 406), the cameras (and associated image-processing functionality of the control and image-processing system 106) are switched into a high-frame-rate, high-power mode, in which motion of an object 114 in contact with surface 116 is continuously tracked (step 408). Frame rates in this mode are typically at least 15 frames per second, and often several tens or hundreds of frames per second. Motion capture and tracking usually continues as long as the object 114 remains in contact with surface 116.

In one implementation, an audio test signal is applied to the surface and a return signal received is by the contact microphone during application of the test signal. The return signal is then compared to stored signals associated with a plurality of media, a stored signal best that matches the return signal is selected, and the audio signals are filtered based on the selected stored signal. When the object 114 leaves the region 112 (as determined, e.g., by the image-processing system 106 based on the motion tracking in step 210), however, control system 206 switches the camera(s) back into low-power mode, and resumes monitoring the contact microphone.

While the technology disclosed has been described with respect to specific implementations, one skilled in the art will recognize that numerous modifications are possible. The number and arrangement of cameras and microphones can be varied. The cameras' capabilities, including frame rate, spatial resolution, and intensity resolution, can also be varied as desired. The microphones' capabilities, including sensitively levels and calibration, can also be varied as desired. Light sources are optional and can be operated in continuous or pulsed mode. The systems described herein provide images and audio signals to facilitate detecting contact of an object with a surface, and this information can be used for numerous purposes, of which reduced power consumption is just one among many possibilities.

Threshold cutoffs and other specific criteria for distinguishing object from background can be adapted for particular cameras and particular environments. Frequency filters and other specific criteria for distinguishing audio signals from background noise can be adapted for particular microphones and particular surfaces. In some implementations, the system can be calibrated for a particular environment or surface medium, e.g., by adjusting frequency filters, threshold criteria, and so on. The use of simple criteria that can be implemented in fast algorithms can free up processing power in a given system for other uses.

Any type of object can be the subject of motion capture using these techniques, and various aspects of the implementation can be optimized for a particular object. For example, the type and positions of cameras and/or microphones can be optimized based on the size of the object whose motion is to be captured, the space in which motion is to be captured, and/or the medium of the surface through which audio signals propagate. Analysis techniques in accordance with implementations of the technology disclosed can be implemented as algorithms in any suitable computer language and executed on programmable processors. Alternatively, some or all of the algorithms can be implemented in fixed-function logic circuits, and such circuits can be designed and fabricated using conventional or other tools.

Computer programs incorporating various features of the technology disclosed may be encoded on various computer readable storage media; suitable media include magnetic disk or tape, optical storage media such as compact disk (CD) or DVD (digital versatile disk), flash memory, and any other non-transitory medium capable of holding data in a computer-readable form. Computer-readable storage media encoded with the program code may be packaged with a compatible device or provided separately from other devices. In addition program code may be encoded and transmitted via wired optical, and/or wireless networks conforming to a variety of protocols, including the Internet, thereby allowing distribution, e.g., via Internet download.

Thus, although the technology disclosed has been described with respect to specific implementations, it will be appreciated that the technology disclosed is intended to cover all modifications and equivalents within the scope of the following claims. 

What is claimed is:
 1. A method of operating an optical motion-capture system with reduced power consumption, the method including: monitoring a contact microphone coupled to an optical motion-capture system, wherein the contact microphone is in contact with a surface of an object upon which one or more cameras of the optical motion-capture system are placed; detecting an audio signal generated by contact of a control object with the object surface; and switching at least one camera of the optical motion-capture system, in response to the detected audio signal, from a standby mode to an operational mode; wherein the standby mode and the operational mode apply to recognition of in-air control object gestures by the optical motion-capture system.
 2. The method of claim 1, wherein the contact is a touch, tap, sequence of taps, or swiping motion of the object.
 3. The method of claim 1, wherein the microphone is a piezo or electromagnetic actuator.
 4. The method of claim 1, wherein the optical motion-capture system includes an ultra-low-power circuit responsive to low-voltage audio signals.
 5. The method of claim 1, further including switching the camera from the operational mode to the standby mode in response to further detection of an audio signal generated by subsequent contact of the control object with the surface.
 6. The method of claim 1, further including filtering the audio signals to exclude frequencies outside a defined range.
 7. The method of claim 1, wherein the contact microphone detects audio signals through the solid object, further including: in response to detection of an audio signal generated by contact of the control object with another object in contact with the solid object, switching the camera of the optical motion-capture system from the standby mode to the operational mode.
 8. The method of claim 1, further including the contact microphone detecting audio signals through the solid object via an intermediate object.
 9. The method of claim 1, further including: monitoring a contact microphone coupled to an optical motion-capture system, wherein the contact microphone detects audio signals propagated through a solid object; and in response to detection of an audio signal propagated through the solid object by a control object, switching the optical motion-capture system from a standby mode to an operational mode.
 10. The method of claim 1, further including: monitoring a contact microphone coupled to an optical motion-capture system, wherein the contact microphone detects audio signals propagated through a solid object; and in response to detection of an audio signal propagated through the solid object by a control object, switching a light source of the optical motion-capture system from a standby mode to an operational mode.
 11. The method of claim 1, further including: analyzing camera images for presence of hands in standby mode.
 12. The method of claim 11, further including: analyzing camera images to determine gestures from tracked motions of hands in operational mode.
 13. An optical motion-capture system, including: a camera for capturing images of an object within a field of view thereof; a contact microphone coupled to the camera and in contact with a surface of an object upon which one or more cameras of the optical motion-capture system are placed; a sensor for monitoring the contact microphone; and a control module for switching the camera from a standby mode to an operational mode in response to detection of an audio signal generated by contact of a control object with the object surface; wherein the standby mode and the operational mode apply to recognition of in-air control object gestures by the optical motion-capture system.
 14. The system of claim 13, wherein the contact is a touch, tap, sequence of taps, or swiping motions of the object.
 15. The system of claim 13, wherein the microphone is a piezo or electromagnetic actuator.
 16. The system of claim 13, wherein the optical motion-capture system includes an ultra-low-power circuit responsive to low-voltage audio signals.
 17. The system of claim 13, further configured to switch the camera from the operational mode to the standby mode in response to further detection of an audio signal generated by subsequent contact of the control object with the surface signifying departure of the control object from a field of view of the camera.
 18. The system of claim 13, further configured to filter the audio signals to exclude frequencies outside a defined range.
 19. The system of claim 13, further configured to include a contact microphone that detects audio signals through the solid object via an intermediate solid object.
 20. The system of claim 13, further configured to: monitor a contact microphone coupled to an optical motion-capture system, wherein the contact microphone detects audio signals propagated through a solid object; and in response to detection of an audio signal propagated through the solid object by a control object, switch the optical motion-capture system from a standby mode to an operational mode.
 21. The system of claim 13, further configured to: monitor a contact microphone coupled to an optical motion-capture system, wherein the contact microphone detects audio signals propagated through a solid object; and in response to detection of an audio signal propagated through the solid object by a control object, switch a light source of the optical motion-capture system from a standby mode to an operational mode.
 22. The system of claim 13, further configured to: analyze camera images for presence of hands in standby mode.
 23. The system of claim 22, further configured to: analyze camera images to determine gestures from tracked motions of hands in operational mode.
 24. A non-transitory computer readable medium storing a plurality of instructions for programming one or more processors to operate an optical motion-capture system with reduced power consumption, the instructions, when executed on the processors, implementing actions including: monitoring a contact microphone coupled to an optical motion-capture system, wherein the contact microphone detects audio signals propagated through a solid object surface upon which one or more cameras of the optical motion-capture system are placed; and in response to detection of an audio signal propagated through the solid object by a control object, switching a camera of the optical motion-capture system from a standby mode to an operational mode; wherein the standby mode and the operational mode apply to recognition of in-air control object gestures by the optical motion-capture system. 